Effect of SiO₂ and Al₂O₃ nanoparticles on the aggregate stability, structure formation, and strength of cementitious materials

Introduction. The use of SiO2 and Al2O3 nanoparticles opens new possibilities for controlling the structure of cementitious materials. However, their influence on the aggregative stability of the coagulation structure and on the strength, characteristics has been insufficiently studied. This study established the effect of SiO2 and Al2O3 nanoparticles on the relationship between aggregative stability, early-age structure formation kinetics, and the strength of cementitious materials. Materials and Methods. Three cementitious systems based on Portland cement CEM I 42.5 were investigated: a reference system with a superplasticizer (Sika®ViscoCrete® 20 HE), as well as systems with the addition of SiO2 nanoparticles (dav = 10 nm, 0.01% by cement mass) and Al2O3 nanoparticles (dav = 50 nm, 0.1% by cement mass). The kinetics of early structure formation were evaluated by plastic strength using a cone plastometer. Aggregative stability was determined by compression rheometry using an Instron 5982 testing machine. The phase composition was established by X-ray diffractometry. The microstructure of the specimens was studied by scanning electron microscopy. Compressive strength was recorded after 1 and 28 days. Results. It was found that Al₂O₃ nanoparticles most significantly accelerate the process of early coagulation-crystallization structure formation in cementitious materials (the exponent of the kinetic equation is 1.13 without a change in the controlling process), increase aggregative stability by a factor of 2–3, but reduce plastic deformation. SiO2 nanoparticles maximally accelerate the hydration process (degree of hydration 93% after 28 days of hardening) while forming low-basic hydrosilicates. The 28-day compressive strength of cementitious materials modified with SiO2 and Al2O3 nanoparticles was 92.7 MPa and 96.4 MPa, respectively, which is significantly higher than that of the reference system (51.1 MPa). Discussion. The twofold increase in strength is attributed to different mechanisms: for SiO2 nanoparticles – their high pozzolanic activity and accumulation of high-strength cementitious phases; for Al2O3 nanoparticles – an increase in the aggregative stability of the coagulation structure, which ensures the formation of a homogeneous crystallization structure without major defects. Conclusion. Complex modifiers based on SiO2 and Al2O3 nanoparticles enable targeted control of both the hydration activity and structural homogeneity of cementitious materials, providing a strength increase of up to 1.9 times.